The production of continuous polyester multi-filament yarns, particularly polyethylene terephthalate (PET) multi-filament yarns, in a two-stage process is already known. Multi-filament yarns are spun and wound during the first stage and, during the second stage, multi-filament yarns are stretched into finished form and thermofixed or else stretch-textured into bulky multi-filament yarns. Between the two stages, the packages of the multi-filament yarns can be stored long-term and transported at elevated temperatures without any influence on the process conditions of the second texturing stage and the quality of the products.
The first step in converting polyester polymer chip to fabric is to make suitable yarn by a spinning process. The most common yarn produced by the spinning process is partially oriented yarn (POY). Experience with texturing processes have led us in the direction of POY yarn which has an elongation greater than 100%. In contrast to PET, yarn produced by the conventional process with polytrimethylene terephthalate (PTT) has resulted in many practical problems. One of the most serious problems that has been encountered is the instability of the yarn on the bobbin on which the final yarn is wound. The instability of the yarn can be observed in various forms including deformed packages, changes in the yarn properties as a function of time, and also changes in the yarn properties as a function of package depth. These problems have limited the use of PTT in further downstream processes.
In contrast to PET multi-filament yarns, polytrimethylene terephthalate (PTT) multi-filament yarns have a considerable shrinking tendency, both immediately after the spinning and upon the winding as well as several hours or days after the winding. This shrinking tendency leads to a shortening of the multi-filament yarns. The yarn package is thereby compressed so that, in an extreme case, the yarn package can no longer be taken off the chuck. During long-term storage or transport, especially at elevated temperatures, the yarn package does not maintain its desired cheese-like shape and forms bulges with hard edges causing not only severe unwinding problems but also leading to a worsening of the yarn characteristics, such as extreme increase of uster values. Only the limitation of the weight of the yarn packages to less than 2 kg provides a remedy for these problems which normally do not occur during the processing of PET yarns.
Furthermore, it has been observed that, in contrast to PET multi-filament yarns, PTT multi-filament yarns age to an increased degree during storage. A structural hardening appears, changing the characteristics of the multi-filament yarns (for example, boil-off shrinkage and degree of crystallization) with time. Industrial use requires that multi-filament yarns maintain their characteristics with time so that subsequent processing of said multi-filament yarns can be carried out continuously and provide multi-filament yarns with constant characteristics.
WO 01/04393 refers to a process wherein the multi-filament yarns are heat-treated by the use of heated godets. Neither the stability in storage nor the stability during transport of the yarn packages obtainable by said method is disclosed in WO 01/04393. A disadvantage of the process of WO 01/04393 consists in that it requires low spinning speeds to be effective. An increase of the spinning speed for economical reasons will decrease the contact time of the multi-filament yarns at the heated godets and therefore result in a decreased long-term stability of the yarn packages.
It would be advantageous to provide a process for the spinning and winding of polyester multi-filament yarn which can be carried out on a large technical scale and in an economical manner. The process in accordance with the invention should permit the highest possible take-up speeds, preferably greater than 2000 m/min.
It would be advantageous to provide to improve the storability of the polyester multi-filament yarn. The yarn should be able to be stored for a longer period of time, such as 11 weeks, for example, as well as to not be adversely affected by elevated temperatures during storage and transport. Shrinkage and deformation of the yarn package during the storage, particularly shrinking to such an extent that the yarn package cannot be taken off the chuck any longer, as well as the formation of bulges with hard edges, should be prevented to the highest extent possible so that no problems of unwinding occur during the subsequent processing of the yarn package.